1. Field of the Invention
The present invention relates to a steering angle detecting device for use in steering control systems for motor vehicles.
2. Description of the Relevant Art
There have recently been developed four-wheel steering apparatus for motor vehicles whererin a four wheel steering mechanism and a rear wheel steering mechanism are not mechanically coupled to each other and also front-wheel steering apparatus for motor vehicles wherein a steering wheel and steerable road wheels are not mechanically connected to each other. Since such a steering apparatus requires the steering angle to be controlled through a feedback loop, it has a steering angle detecting device which detects the steering angle of the steering wheel and the steering angle of front or rear road wheels. In general, the steering angle detecting device has been in the form of an analog sensor such as a differential transformer or a digital sensor such as an increment-type rotary encoder as disclosed in Japanese Laid-Open Utility Model Publication No. 60-43473, for example.
The analog sensor is however problematic in that its output characteristics are easily subject to changes due to fluctuations in the power supply voltage and changes in the temperature, resulting in low detecting accuracy. If the analog sensor is small in size, its output voltage (indicated by the solid-line curve in FIG. 7 of the accompanying drawings) deviates from the ideal linear pattern (indicated by the broken-line curve) in a larger steering angle range. Therefore, linear output voltage pattern is not maintained and the detecting accuracy is low in the larger steering angle range.
The digital sensor is advantageous in that it is immune to disturbances such as voltage fluctuations and hence has a high detecting accuracy. However, since pulses produced at the time the steering angle varies are counted by a counter, when the ignition key is turned off, or the battery is removed for maintenance or otherwise the power supply is turned off, the count of the counter must be stored in a memory, and the memory adds to the cost of the digital sensor. In order to solve the above problem, there has been proposed a digital sensor which has a means for generating a reference signal in the steering neutral position. The proposed digital sensor however fails to detect the steering angle until the steering angle reaches the reference position, and therefore is not suitable for the feedback control of the steering angle in a steering apparatus which is required to control the steering angle highly accurately throughout a full operation range beginning from an initial stage of operation.